Table 2.

Frequently-used fish vaccine types.

Classification		Merit	Defect
Live vaccine		Generally attenuated vaccine; Close to natural infection; Effectively stimulate immune system; Pathogen can reproduce in vivo; Low dosage; Long protection duration; No adjuvant needed	Poor safety under natural conditions; Short shelf life; Not convenient storage and transportation
Inactivated vaccine		Short development cycle; Safe to use; Easy to preserve	Cannot reproduce in vivo after immunization; Large immunize dosage; Short duration; Appropriate adjuvant is needed
Gene engineering vaccine	Recombinant subunit vaccine	Excellent safety; Simple production; Easy control; High stability; High purity	The expression is affected by the expression system; Weak immunogenicity; Short protection duration; Difficult to effectively remove intracellular pathogens
	DNA vaccine	High stability; Induce comprehensive immune response; Easy production in large scale; No risk of return of the virus; Low cost; The most promising vaccine	Uncertain immune response caused by the vaccine; Tissue distribution and expression still unknown; More researches are needed to confirm the stability; Immune tolerance; Risk of integrating into the genome
	Gene deletion/mutant vaccine	High stability; Not easy to reverse under natural conditions; Good immunogenicity; Guaranteed safety; Important direction of vaccine development	Not many successful cases;
	Living-vector vaccine	Can be administered through mucous membrane system; Easy control;	Restricted by its expression level and its persistence; Risk of plasmid loss reducing vaccine function; Environmental safety needs to consider